Female pelvic organ prolapse (POP) is a disabling, chronic condition in which the uterus and vaginal walls protrude through the vagina, causing discomfort as well as bladder and bowel dysfunction. A woman's lifetime risk of POP surgery is 19%. Prevention strategies have not been identified because the biological mechanisms underlying POP are uncertain. Other than aging, the only known risk factor for POP is vaginal childbirth. Specifically, prior research by this team demonstrated that vaginal delivery is associated with a 5-fold increase in the odds of POP. However, the mechanisms for this association remain unknown. Studies proposed in this application will investigate whether obstetrical levator ani muscle injury (LAMI) contributes to POP through alterations in the structure and/or the function of the levator muscles. LAMI, which occurs after 20% of vaginal births, can be detected with 3-dimensional ultrasound imaging. The central hypothesis of this proposal is that LAMI leads to POP through two discrete mechanisms: chronic widening of the levator hiatus (change in structure) and weakness of the levator muscle complex (change in function). The specific aims of this proposal are: (1) to compare levator ani muscle structure and function between women with and without LAMI, as identified on 3D ultrasound; (2) to determine the relative contributions of LAMI, levator ani muscle structure and function to POP. To accomplish these aims, 460 vaginally parous women from this team's established longitudinal cohort study (R01HD056275) will be evaluated 5-15 years from delivery. Evaluation will include quantitative assessment of pelvic organ support, 3D imaging of the levator muscle complex, and objective measures of levator strength. 3D ultrasound is an exciting tool that provides dynamic, real time views of the levator muscle and will enable us to detect LAMI and to distinguish structural and functional differences between those with and without LAMI. Thus, the proposed studies will consider not only the role of LAMI in the development of POP, but also the long term sequelae of LAMI (such as chronic changes to the levator hiatus). LAMI has emerged as an important obstetrical complication that is currently not preventable, except by elective cesarean birth. This research will be significant because POP prevention will not be possible until the pathophysiology is elucidated and markers for POP risk are identified. The studies proposed in this application are expected to provide markers that will identify women at greatest risk for POP and will serve as a platform for developing effective prevention strategies. Given the high prevalence of POP, prevention is critical to reduce the public health burden.